Invisible objects ruining my noobgame :D

Hello and sorry for bothering you with this noobish question but ive searched for a few hours without any luck so ill give it a try.

I've just started learning Blender and trying to do a basic game (a block moving on some other blocks). But the movement are sometimes interrupted by invisible objects either stoping the "car" or making it turn... I dont understand what it is? I mean there cant be anything since its just two basic blocks but still...

I'm not entirely sure what the problem is, but the problem seemed to lessen significantly when I made the Margin in the Collision Bound section for both the car and the terrain to 10cm. Other than that, I don't know how you could definitively get rid of it. Even at 5cm for both its a little bumpy...I don't know a heck of a lot about the game engine, so I can't really help much beyond that.